Solvent Effects on the Rate of Heterolysis of t-Butyl Chloride, Bromide, Iodide, and 2,4-Dinitrophenolate

Article abstract of DOI:10.1246/bcsj.67.824

Rates of heterolytic decompositions of t-butyl chloride, bromide, iodide, and 2,4-dinitrophenolate have been measured by an NMR method in eight deuterated or undeuterated polar solvents; methanol-d₄, ethanol-d₆, dimethyl-d₆ sulfoxide, N,N-dimethylformamide-d₇, acetonitrile-d₃, pyridine, nitrobenzene, and acetone-d₆.The observed solvent effect is discussed on the basis of cation and anion solvation.With t-butyl halides, the anion solvation due to hydrogen bonding by protic solvents drastically decreases on increasing the radius of halide ions, and thus the differential solvation transferred from anion-solvating methanol to cation-solvating dimethyl sulfoxide is dramatically reversed on going from the chloride to the iodide (k_Me2SO-d6/k_methanol-d4 at 60 deg C: t-BuCl, 5E-2; t-BuBr, 6E-1; t-BuI, 7).The 2,4-dinitrophenolate behaves like the iodide.Complicated products are obtained in the decomposition of t-butyl bromide in dimethyl sulfoxide; however, a mechanism involving a bimolecular decomposition process of the bromide is ruled out.During the decomposition od t-butyl iodide in dimethyl sulfoxide, an intermediate salt t-butoxydimethylsulfonium iodide has been detected.The salt decomposes to isobutene, but the process is much slower than the heterolytic decomposition of t-butyl iodide itself at ambient temperature.